An Independent Evaluation of an Alternative Rapid Method for the Enumeration of S. aureus in Soy Ingredients

Introduction: The presence of S. aureus in foods poses several problems to the food industry, as they may indicate post-process contamination of the commodity and certain strains of the organism are known to produce a potent enterotoxin that can be a public health hazard.  Rapid quantitative microbiological methods offer a variety benefits including increased turnaround time, reduced labor cost, and a streamlined laboratory process.  These methods are typically validated against the reference method through a recognized validation organization.   However, these validations are carried out on limited number of food product and it is important for an end user to conducting their own independent evaluation.

Purpose: The purpose of this evaluation was to proactively evaluate the performance of the AOAC OMA 2003.07 method against the FDA BAM Chapter 12 on a variety of low moisture, soy ingredients: Isolated Soy Protein (ISP), Soy Fiber (SF), Soy Fluid Lecithin (SFL), Soy Deoiled Lecithin (DL), and Soy Nuggets (SN). 

Methods: For each matrix, 5 replicates at three contamination levels (10-100 CFU/g, 100-1,000 CFU/g and 1,000-10,000 CFU/g) were evaluated.  Three uninoculated control replicates were included for each matrix.

Results: Results for each contamination level were analyzed for the difference of means and reverse transformed difference of means (including 95% confidence intervals).  The results of the study indicated that 7 out of 18 contamination levels evaluated produced statistically significant results between the alternative and reference method (indicated by a confidence interval that falls outside of -0.5 to 0.5).

Significance: The data generated in this evaluation indicates that the rapid alternative method may not be a suitable alternative method for the detection of S. aureus in these select commodities.  The results of this study confirm the importance to the food industry to conduct their own internal verification of matrices not included in the scope of a method during the validation process.